Centrifugal pumps are frequently used to pump slurries consisting of a finely divided solid suspended in a liquid. Due to the erosive action of the pumped slurry on the tips of the impeller, it is necessary to limit the operation speed of the centrifugal pump. In practice, it has been found that the speed of the impeller tip must be limited to approximately 120 feet per second. This limitation on the tip speed limits such conventional centrifugal pumps to low pressure applications. Also, when the conventional centrifugal pump is used to pump slurries containing abrasive material, such as coal, a great deal of wear occurs in the periphery of the rotor, and necessitates the replacement of the entire pump, or if the periphery of the impeller is replaceable as pointed out in U.S. Pat. No. 4,076,450, only the worn parts need to be replaced. However, such replacement is still required too frequently and the lost time and labor for repair add considerably to the expense of operating such pumps.
Another wear problem in centrifugal pumps of the volute type is bearing and packing wear. In such pumps the radial thrust is only uniform at the optimum design speed of the pump. At lower speeds, particularly when the pump is started or is stopped, the radial thrust is non-uniform. Due to this non-uniform thrust condition attempts have been made to stiffen the support assembly and to compensate for the effect of the thrust by complex bushing designs. See U.S. Pat. No. 4,224,008 in this regard.
For higher pressure, a number of centrifugal pumps can be cascaded. U.S. Pat. No. 4,239,422 shows such an arrangement. Since failure of any single pump in such an arrangement is possible and would cause the total system to fail, such a system has low reliability. To improve reliability, it would be preferable to use a single pump instead of the cascaded centrifugal pumps, but this is not possible with the conventional centrifugal pump.
Positive displacement type pumps, such as reciprocating plunger pumps, can be used in high head applications, but due to abrasion wear, are unsatisfactory with high abrasive slurries. Such high abrasive slurries cause unacceptable rapid wear on check valves and packings.
U.S. patent application Ser. Nos. 188,047, 032,651, 199,861, and 036,843 disclose apparatus for pumping a dry pulverized material in a high head situation. Although these disclosed pumps are adequate for pumping a dry material, they are not suited for pumping slurry mixtures.